Accelerator Mass Spectrometry (AMS) is a high-energy isotope mass spectrometer based on accelerator technology and ion detector technology and is mainly used for the measurement of isotope abundance ratio. By virtue of an accelerator, the current AMS accelerates and measures isotopes sequentially and alternately thereby analyzing the isotopes. Thanks to the use of an accelerator and a detector, AMS is capable of excluding molecular ion background and isobaric ion background, which has greatly improved the analytical sensitivity and, as a result, the isotope abundance sensitivity can reach up to 1×10−15. In contrast, the prior-art mass spectrometer (MS) only has an isotope abundance sensitivity of 1×10−8 due to the interference from molecular ion background and isobaric ion background.
Although the AMS is advantageous in that it has a high sensitivity and requires a smaller sample amount, it is more complex in structure than the ordinary MS. Further, as isotopes are injected and measured alternately, the AMS cannot measure the isotopes simultaneously. These have contributed to undesirable measurement accuracy of the AMS, generally around 1%-3%.
The advantages and disadvantages of AMS and MS are shown in the table below:
AdvantagesDisadvantagesAMSThe abundance sensitivity isIsotopes are injected andas high as 10−15; the amountmeasured alternately; theof samples required is lessaccuracy is not highthan 0.1 mg.enough, around 1%-3%.MSMore isotopes are receivedThe abundance sensitivityand the accuracy isis not high enough (10−8).0.1%-0.5% higher.
The main reason why AMS cannot be used for measuring isotopes simultaneously lies in that, since the application of accelerator from the 1940s, it has been the practice that the accelerator can only accelerate a nuclide ion at a time. The accelerator system consists of an ion injector, an accelerator and a high-energy ion analyzer. One of the main components in the injector is an injection magnet which is intended to select one isotope and injects it into the accelerator for acceleration. To allow more than two isotopes to be measured, the mass parameter of the injector must be alternately changed so as to inject and accelerate the isotopes alternately thereby measuring the isotopes alternately.
Due to alternate measurement of isotopes, two major problems occur with the AMS. First, the measurement accuracy is not high enough, generally about 1%-3%; second, the instrument system of the AMS is more complicated and, as compared with conventional MS, an injection magnet, an alternate injection power supply and a control system in addition to an accelerator are included.